Electron beam gun

ABSTRACT

An electron beam gun is described wherein a pair of elongated filaments are placed parallel to one another in the elongated channel of a focusing electrode.

n munill/M31472 Kl, 3 701 915 0 Umted States Patent 1151 3,701,915Tsujimoto 1 Oct. 31, 1972 [S4] ELECTRON BEAM GUN [56] References Cited[72] Inventor: galz fimi N. Tsujimoto, El Cerrito, UNITED STATES PATENTS3. l 2,229,752 l/l94l Jonker et a1. ..3l3/70 [731 Asslgnee: g s? 5 5' 3"2,090,722 8/1937 Bouwers ..313/70 x ew 1 2,301,743 11/1942 Nagy et a1..313/70 x [22] Filed: Jan. 4, 1971 Primary Examiner--Herman KarlSaalbach [21] Appl' NO" 103684 Assistant ExaminerSaxfield Chatmon, Jr.

Att0meyFitch, Even, Tabin & Luedeka [52] us. c1 ..313/82, 219/121 EB,313/70,

0 213/343 57 ABSTRACT [51] Int. Cl .1101; 29/46, H01 1 1/00, H01 29/56[58] Field of Search ..313/70, 82, 343; 219/121 EB An electron beam s 18descrlbed wherem a P of elongated filaments are placed parallel to oneanother in the elongated channel of a focusing electrode.

3 Claims, 3 Drawing Figures PATENTED I973 3.701. 915

INVENTOR.

KAZUMI N. TSUJIMOTO BY find/ma iw/ala fa ATTORNEYS ELECTRON BEAM GUNThis invention relates to electron beam guns such as are used in highvacuum electron beam furnaces. More particularly, the invention relatesto an electron beam gun utilizing improved means for producing theelectrons.

Electron beam furnaces have been used for some time in the vacuumprocessing of various materials. Such furnaces are utilized, forexample, in the melting and casting of metallic ores to obtainrelatively pure metals or alloys. Such furnaces are also used in themelting of materials other than metals, such as ceramics and plastics,and are frequently used to produce vapors of metals and other materialsfor-deposition upon a substrate.

Electron beam furnaces utilize one or more electron beam guns forproducing high energy electron beams. These beams are then directed insome manner to a target for heating the same. Electron beam gunsgenerally comprise a heated electron source or emitter for emitting theelectrons, and suitable means for generating a magnetic field foraccelerating and focusing the electrons into a beam. The interior of thefurnace is usually evacuated to a high degree and the electron beam gunis disposed at a convenient location within the vacuum chamber.

A particularly advantageous type of electron beam gun utilizes anelongated filament or emitter (of tungstem or other suitable material)disposed in an elongated channel in a backing or focusing electrode. Thefilament is heated to a thermionically emissive temperature and thebacking electrode is maintained at a suitable negative potential inorder to direct the electrons produced by the filament out of the openside of the channel. Suitable means may then be provided foraccelerating and directing the electrons to a target. The beam producedis typically of a ribbon shape.

In electron beam furnaces, many ions may be produced as a result ofvaporization of various materials in the furnace. The filaments oremitters of electron beam guns of the type described are susceptible toerosion as a result of bombardment by such ions. This is becausepositively charged ions may be attracted to the negatively chargedbacking electrode of the electron beam gun and thereby collide with thefilament of the gun, producing erosion. The more frequently it becomesnecessary to replace the filament of an electron beam gun, the moreinefficient the apparatus in which it is incorporated becomes.

It is therefore an object of the present invention to provide animproved electron beam gun such as may be used in an electron beamfurnace.

Another object of the invention is to provide an electron beam gunhaving long filament life.

Still another object of the invention is to provide an electron beam gunwith less susceptibility to filament erosion due to ion bombardment.

Other objects of the invention and the various advantages thereof willbecome apparent to those skilled in the art from the following detaileddescription taken in connection with the accompanying drawings wherein:

FIG. 1 is a side elevational view of an electron beam gun constructed inaccordance with the invention;

FIG. 2 is s top view of the electron beam gun of FIG. 1, with the upperelements removed; and

FIG. 3 is a partial sectional view taken along the line 3-3 of FIG. 1.

Very generally, the electron beam gun of the invention includes afocusing electrode 11 defining an elongated open-sided channel 12. Means13 are provided for connecting the focusing electrode to a first sourceof potential. At least one accelerating electrode 14 or 15 is spacedfrom the open side of the channel defined by the focusing electrode.Means 16 are provided for connecting the accelerating electrode to asecond source of potential which is substantially more positive than thefirst source. A pair of elongated filaments l7 and 18 are provided ofthermionically emissive material. The filaments are supported parallelwith each other and within the channel extending along the lengththereof and spaced from the focusing electrode. The filaments may beconnected to a source of heating current to raise them to thermionicallyemissive temperatures.

Referring now more particularly to the illustrated embodiment of theinvention, it may be seen that the electron beam gun of the inventionutilizes two parallel emitters or filaments l7 and 18. The emitters orfilaments are each comprised of a thin tungsten wire, such as a wire ofapproximately 0.080-inch diameter. The filaments may be machined withopposite flattened surfaces, not shown, in which case the length of theflattened surfaces will correspond to the width of the ribbon beam it isdesired to produce. Heating current conducted through the filamentscauses emission of free electrons by the heated tungsten filaments inaccordance with known phenomena.

The focusing electrode 11, which is maintained at a negative potentialwith respect to the accelerating electrodes subsequently described, isdisposed immediately adjacent the two parallel filaments l7 and 18. Thefocusing electrode 11 is provided with the open-sided elongated channel12 which, as may be seen from FIG. 3, comprises a generally flat bottom21 and a pair of planar sides 22 and 23 extending therefrom and diverging from each other. The parallel filaments 17 and 18 are supportedwithin the channel extending along the length thereof and are spacedfrom the focusing electrode 11. Preferably, the filaments are supportedon opposite sides of the longitudinal axis plane of the channel, thatis, a plane extending along the length of the channel perpendicular tothe bottom 21 thereof and extending therefrom midway between theintersection of the bottom with the sides 22 and 23. The focusingelectrode serves to cause the electrons emitted by the filaments to bedirected out of the open side of the channel 12.

In order to accelerate the electrons moving out of the open side of thechannel 12, the electron beam gun illustrated is provided with a pair ofaccelerating electrodes l4 and 15 in the form of a pair of parallel bars(not shown in FIG. 2). The accelerating anodes or bars 14 and 15 aresupported between a pair of parallel connecting plates 16 which alsoserve as the means for connecting the accelerating electrodes to asource of potential which is substantially more positive than the sourceof potential to which the focusing electrode 11 is connected. As aresult, electrons moving out of the open side of the channel 12 areaccelerated into a beam of a generally ribbon shape. As an alternative,the

anodes may comprise screens or grids which produce accelerating fieldsbut which permit the beam to pass therethrough, or the target itself maycomprise an accelerating anode.

The focusing electrode 11 has a downwardly extending appendage 27 forsupporting the focusing electrode. The appendage 27 is secured to amounting plate 13 by means of mounting bolts 29 passed through suitableopenings in the mounting plate and threaded into the appendage 27. Themounting plate 13 may also serve as the means for connecting thefocusing electrode to a source of potential which is negative withrespect to the accelerating electrodes 15 and 16. Further holes, notshown, may be provided in the mounting plate 28 to facilitate securingthe mounting plate to a suitable mounting bracket, also not shown, inthe electron beam furnace. Thus, the electron beam gun, depending uponthe orientation and position of the unillustrated mounting bracket, maybe disposed to direct the beam to almost any position in the electronbeam furnace.

Opposite ends of the support plate 13 are each provided with adownwardly extending arm 31 and 33, respectively. The arms 31 and 33 arepreferably made integral with the support plate 13. The downwardlyextending arms 31 and 33 provide support for a pair of spacer blocks 34and 36, to which the means which support the filaments or emitters areattached. The spacer blocks 34 and 36 are of electrically insulatingmaterial and are attached to the arms 31 and 33, respectively, bymounting bolts 37 and 38. The bolts 37 are secured by suitable nuts 39and the bolts 38 are secured by suitable nuts 41.

Turning now to the means by which the emitters or filaments l7 and 18are supported, two conductive members in the form of leaf springs 42 and43 are secured to the spacer blocks 34 and 36 on the arms 31 and 33.Both of the leaf springs 42 and 43 extend from the spacer blocks 34 and36 on the opposite sides of the focusing electrode 11 and terminate nearthe ends of the filaments 17 and 18.

The leaf springs 42 and 43 are made of electrically conductive materialin order to conduct heating current to the emitters supported thereby,explained subsequently. This heating current is conducted to the leafsprings by a plurality of suitable electrical connectors 46 andconductive cables 47 connected thereto. The electrical connectors arebolted in contact with the corresponding leaf springs by means of thesame bolts which secure the leaf springs against the spacer blocks 34and 36. Each of the electrical connectors is provided with a socket 48in which one end of the cable associated therewith is received.

The ends of the leaf springs 42 and 43 nearest the focusing electrode 11are provided with recesses 49 and 51 therein. These recesses are formedin the leaf springs, such as by hot stamping, and each recess comprisesa pair of intersecting planar surfaces. Thus, each recess has a roughlyV-shaped cross section. Two parallel slots 52 are provided in the endsof each of the leaf springs. Each slot extends past the apex orintersection line of the planar surfaces comprising the recess and is ofa size sufficient to permit the filament or emitter associated therewithto pass through the slot to the opposite side of the leaf spring.

Cylindrical contact elements 53, 54, 55, and 56, respectively, aresecured to each end of each of the filaments or emitters 17 and 18. Allthe contact elements are electrically conductive and are provided withsuitable holes therein through which the associated emitter passes. Thecontact elements are secured to the emitter by suitable means such as ashrink fit, welding, soldering, etc. The contact elements have curvedsurfaces which mate with the corresponding recesses in the ends of theleaf springs. Alternatively, a construction may be used wherein thesurfaces of the elements have V- shaped protrusions which mate with thecorresponding recess. In either case, good contact exists between eachleaf spring and the contact element in engagement therewith. As may beseen in FIG. 1, the center line of the emitters is preferably on aradius of the curved surface of the contact elements.

Although the illustrated embodiment utilizes cylindrical contactelements which mate with V-shaped recesses, this configuration is notcritical. For example, where the contact elements are cylindrical inshape as shown, the recesses may be formed to have correspondingcylindrical surfaces. The basic consideration is to provide a matingconfiguration which establishes satisfactory electrical contact betweenthe emitter and the conductive springs, and which provides sufficientsupport for the emitter as explained in detail subsequently. Also, wherethe emitters are provided with flat surfaces to increase the surfacepresented to the direction the beam is to travel, the configurationshould facilitate alignment of the flat area of the emitters withrespect to the rest of the gun.

A pair of guard straps 57 are provided at the ends of the emitters l7and 18. The guard straps are attached to the respective electricalconnectors 46 on the opposite sides thereof from the respective leafsprings. The guard straps 57 are preferably of conductive material andare provided with holes therein near the ends of the guard straps closeto the emitters such that the ends of the emitters projecting beyond thecontact elements extend through the holes. This ensures that the emitterwill not fall and short to ground in the event it breaks.

The filaments or emitters, being thin tungsten wires, are easily flexedor bent. This fact makes it important that the emitters be maintainedunder a slightly taut condition to ensure that the spacing between theemitters and the focusing electrode is constant for a constant electronbeam configuration. In order to accomplish this, the leaf springs arebiased outwardly of each other an amount which applies tension on eachof the emitters which is sufiicient to maintain the emitter taut butwhich is below the yield point of the emitter material at operatingtemperatures. Thus, the emitter will not be damaged or pulled apart dueto excessive tension. The particular configuration of the inventionenables the proper tension to be readily achieved by balancing springbias while at the same time provides adequate electrical contact forconducting heating current through the emitter. Satisfactory resultshave been achieved in apparatus of the illustrated type using tensionsin the range of 220 to 400 grams at operating temperatures of 2,200C to2,300C.

As previously mentioned, normal electron beam furnace operationgenerally necessitates occasional replacement of the emitters ofelectron beam guns. A simple manual pressure on the leaf springssupporting the emitter will relieve the outward bias of the leaf springssufficiently to permit removal of the emitter. When the manualcompression on the springs is released, the bias of the springs willposition the emitter and support it as above described. In the event oneof the filaments is longer than the others, a slight manual twisting maybe imparted to the leaf springs 17 or 18 to permanently deform thesprings and take up whatever difference in length is necessary.Alternatively, two separate pairs of supporting leaf springs may beutilized to support the respective filaments.

Because of the natural focusing tendency of the electrostatic fieldestablished by the focusing electrode 11, ions moving into the elongatedchannel 12 through the open sides thereof are focused and directed topass between the parallel filaments 17 and 18. The ions therefore hitharmlessly in the bottom of the channel and do not strike either of thefilaments to cause erosion. Thus, a substantial improvement in filamentlife is effected. Naturally, with the additional filament, greater beampower may also be provided since there is more emissive surfaceavailable to produce electrons.

A natural redundancy is also provided by the electron beam gun of theinvention. Even though ion bombardment is minimized, there is a naturaltendency for the filament to neck down or become smaller with the lossof electrons over the period of their operating life. As one of thefilaments begins to neck down, however, its electrical resistance goesup and more current is therefore shunted to the neighboring filament,resulting in a natural correction and even life for both filaments.

A further advantage provided by the invention is that a more uniformenergy density beam is produced without a substantial increase in thecross sectional size of the beam over that which would be produced by asingle filament gun. The particular advantage of this feature is that ofovercoming problems in evaporation within an electron beam furnace dueto localized high energy density at the target which leads to spittingand localized high pressures. The more uniform energy density of thebeam minimizes this possibility.

The depth within the channel 12 which the filaments l7 and 18 areplaced, and the spacing between the filaments and the spacing betweenthe focusing electrode 11 and the accelerating electrodes 14 and 15 areall generally interdependent. Moving filaments deeper into the channelreduces the perveance of the gun and decreases the thickness of theribbon beam. Moving the filaments further out toward the open side ofthe channel results in an increase of the beam thickness, the latterbeing limited to a thickness less than that which would cause theelectrons to strike the accelerating electrodes. One satisfactory designutilizes an anode to anode spacing of 1 inch center to center, an anodeto cathode spacing of 0.450 inch, a filament depth of 0.145 inch, afilament to filament separation of three sixteenths inch, a channeldepth of one-quarter inch with an average channel width equal to itsdepth. Such a gun produces a broad beam at 30 KV anode to cathodepotential.

It may therefore be seen that the invention provides an improvedelectron beam gun of substantially increased filament life. The gun isesi gned to prevent excesslve ion bombardment o te filaments andproduces a beam of more uniform energy density than is the case with asingle filament gun.

Various modifications of the invention will become apparent to thoseskilled in the art from the foregoing description and accompanyingdrawings. Such modifications are intended to fall within the scope ofthe appended claims.

What is claimed is:

1. In an electron beam furnace, an electron beam gun comprising, afocusing electrode defining an elongated open-sided channel, means forconnecting said electrode to a first source of potential, at least oneaccelerating electrode spaced from the open side of said channel definedby said focusing electrode, means for connecting said acceleratingelectrode to a second source of potential which is substantially morepositive than said first source, a pair of elongated filaments ofthermionically emissive material, means for supporting said filamentsparallel with each other within said channel extending along the lengththereof and spaced from said focusing electrode, said filaments beingpositioned on opposite sides of the longitudinal axis plane of saidchannel with the space between said filaments being unobstructed, andmeans for connecting said filaments to a source of heating current.

2. An electron beam gun according to claim 1 wherein said focusingelectrode is a solid body with said channel comprising an elongatedrecess in a surface thereof.

3. An electron beam gun according to claim 1 wherein said channel iscomprised of a generally flat bottom and a pair of planar sidesextending therefrom and diverging from each other.

1. In an electron beam furnace, an electron beam gun comprising, afocusing electrode defining an elongated open-sided channel, means forconnecting said electrode to a first source of potential, at least oneaccelerating electrode spaced from the open side of said channel definedby said focusing electrode, means for connecting said acceleratingelectrode to a second source of potential which is substantially morepositive than said first source, a pair of elongated filaments ofthermionically emissive material, means for supporting said filamentsparallel with each other within said channel extending along the lengththereof and spaced from said focusing electrode, said filaments beingpositioned on opposite sides of the longitudinal axis plane of saidchannel with the space between said filaments being unobstructed, andmeans for connecting said filaments to a source of heating current. 2.An electron beam gun according to claim 1 wherein said focusingelectrode is a solid body with said channel comprising an elongatedrecess in a surface thereof.
 3. An electron beam gun according to claim1 wherein said channel is comprised of a generally flat bottom and apair of planar sides extending therefrom and diverging from each other.